[Amps] Alpha 76PA Conversion - Drive Issue

[gudguyham at aol.com]

3cx800 tubes like 2600-2700 on the plates for maximum gain.   2000 volts is a bit anemic for 3cx800 tubes.  That modification was good when 3cx800 tubes were plentiful.  They are going in the same direction as 3cx400’s did now and the modification doesn’t make much sense anymore.  I’d settle for a used Commander these days..  


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On Tuesday, November 30, 2021, 4:09 PM, Steve London <n2icarrl at gmail.com> wrote:

6 years ago, I converted a 3x8874 Alpha 76PA to a pair of 3CX800's. The
tubes were the Chinese ones that Alpha was selling for a while. Even after
removing the resistor pack in the input circuit, I had trouble getting more
than 1300 watts output without backing off on the loading and running with
the grid current near the trip out point. I always blamed it on the Chinese
tubes.

Today I did a science experiment. First, I put the input network back to
the standard Alpha 76PA "low drive" configuration with the 5x470 ohm
resistor pack. Then I borrowed the 3CPX800's from my Commander HF-2500M.
With those tubes, it's no better, and maybe even worse than the Chinese
3CX800's. These same 3CPX800's work wonderfully in the Commander with 1500
watts output at 35-45 watts drive. (Yes, the Commander runs them at a
higher plate voltage, and has a tuned input circuit on each band). Here are
the specifics with the 3CPX800's in the Alpha 76PA:

Drive: 95 watts
HV: 2000 volts
Ip: 880 mA
Ig: 65 mA
Po: 1260 watts
Efficiency: 71.5 %

Seems like I should be able to get higher Po with less drive.

73,
Steve, N2IC
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